Abstract
Chapter 2 Experimental details
2.3 Pulsed laser deposition (PLD) set-up
Pulsed laser deposition technique is simple yet versatile which can be used for deposition of thin films of complex materials with high quality. The pulsed laser deposition (PLD) belongs to the family of physical vapor deposition (PVD). In this technique, a high power pulsed laser is focused on to a sintered target furnishing the target plasma plume which expands in the background ambient and deposited on to the substrate placed few cm apart from the target. The basic mechanisms involved in the PLD process, are classified into three steps (a) laser-matter interaction resulting into ablation
Chapter 2: Experimental details and vaporization of target material and formation of laser induced plasma (LIP), (b) interaction of LIP with the incident laser beam via inverse-bremsstrahlung as well as expansion and cooling of plasma and (c) growth of the film onto the substrate.
Laser ablation involves absorption of laser radiation by the target material leading to electronic excitation, breakdown and plasma formation, etc. For creation of LIP, a minimum focal intensity of the order of ~108 W/cm2 is required. Generally, nano-second lasers: Q switched Nd: YAG and excimer lasers, are capable of delivering such a high intensity [1]. The ablation from the target material depends on the properties of the material such as absorption coefficient (), reflectivity (R), specific heat (Cv) and thermal conductivity (K) etc. It also depends on the laser parameters (laser energy, wavelength, pulse width). In nanosecond laser ablation, the laser pulse width is larger as compared to that of the time interval of entire interactions taking place inside the material. In the initial stage of the laser pulse, the absorption of laser energy by the material leads to the ablation and breakdown of the material. The LIP consists of neutral atoms, ions and electrons etc.
In the second stage, the plasma expansion takes place under ambient gas. During the expansion of laser plasma, it absorbs the laser energy via inverse bremsstrahlung process causing further ionization and increases the surrounding temperature. The pressure of the initially formed plasma plume near the target is very high at the initiation of plasma which commences with the nearly solid density (of the target) along with very high kinetic energy (K.E.) of the emitted particles. This generates a pressure gradient between the plasma plume near the target and the ambient environment (~10-6-10-1 mbar) which results in the expansion of plasma plume with an expansion velocity of ~103-105 m/sec.
During the plasma expansion, it gets cool down and simultaneously the interaction with the ambient gas leads to the formation of molecular species if applicable (e.g. formation
Chapter 2: Experimental details of oxide/nitrides etc.). In the final stage, as a result of condensation and nucleation of the molecular species, the formation of thin film onto the substrate takes place. The nucleation and growth of the film is dependent on the dynamics of the LIP, the surface energy of the substrate, etc.
Conventionally PLD is viewed to be limited to deposition over small area (~1-2 cm2) only. But this limitation is overcome by incorporating multiple laser beams [3] as well as by translating the substrate and laser beam suitably [4]. Another problem with this technique is the ejection of large particulates (in the form of liquid droplets or clusters) from the target and being deposited directly on the substrate thus rendering the poor surface quality. But, this problem can be overcome by carefully optimizing the deposition parameters in particular the laser fluence, background pressure and target to substrate distance [2, 5, 6].
There are some major advantages of PLD over other fabrication techniques. One of the salient features of PLD is the easy control over deposition parameters depending on the properties of the target and the required thin films. Another major advantage of PLD is its ability to transfer the stoichiometry from the target to the deposited thin film. The fabrication of highly crystalline thin film of even very complex material can easily be achieved at relatively low substrate temperature by this technique as compared to those of other thin film deposition techniques.
The schematic diagram of the pulsed laser deposition (PLD) setup used for deposition of Zn1-xAlxO (0≤x≤0.10) and Zn1-xTixO (0≤x≤0.05) thin films in the present work is shown in Fig. 2.2. A beam from the second harmonic of a high power Q-switched Nd:YAG laser (Quanta System, HYL-101, ~10 ns, 10 Hz) was focused from outside, after steering itsuitably, with high damage threshold right angled prism (not shown in
Chapter 2: Experimental details
Fig. 2.2 Schematic diagram of pulsed laser deposition (PLD) chamber.
Fig. 2.2), onto the target placed inside the PLD chamber by a plano-convex lens of 35 cm focal length. Prior to the deposition, the PLD chamber was evacuated to a base pressure of 2.5×10-5 mbar by a turbo molecular pump (Pfeiffer, Hi Pace 300 C) backed by rotary pump (Pfeiffer, DUO 10MC). The turbo pump was connected to the bottom of the PLD chamber through a 100 CF port. The PLD target (sintered pellets) was mounted onto motorized target carrousel which was installed inside the PLD chamber through one of the 150 CF port. The substrate (fused silica and Si) was mounted on the substrate holder through another 150 CF port opposite to the target port of the PLD chamber. The substrate holder was equipped with resistive heating to maintain the desired temperature during deposition. All the thin films were deposited at a substrate temperature of ~500 0C.
The substrate to target distance during fabrication for all the thin films was at kept at ~3 cm. The deposition of the films was carried out under an oxygen environment, at a
Chapter 2: Experimental details pressure of 10-1 mbar. The pressure inside the chamber was monitored by a compact cold cathode gauge (Pfeiffer, IKR 251) and pirani gauge (Pfeiffer, PCR 280 and Hind HiVac, HPS-2) working in the low pressure regime (10-2-10-7 mbar) and high pressure regime (103-10-3 mbar) respectively. The thin films were deposited at a laser fluence of ~10 J/cm2. All the thin films were deposited for 30 minutes duration.
All these parameters for depositing the Zn1-xAlxO (0≤x≤0.10) and Zn1-xTixO (0≤x≤0.05) thin films are listed in table 2.3.These are the optimized parameters for fabrication of pure ZnO thin film via PLD [7].
Table 2.3 Deposition parameters for Zn1-xAlxO (0≤x≤0.10) and Zn1-xTixO (0≤x≤0.05) thin films via PLD.
Serial No. Parameters Numerical Value
1. Ambient Pressure 2.5×10-5 mbar
2. Oxygen Pressure 10-1 mbar
3. Substrate Temperature 500 ˚C
4. Laser Fluence 10 J/cm2
5. Substrate to Target Distance 3 cm
6. Deposition Time 30 minutes
7. Substrate Fused Silica, Silicon
Chapter 2: Experimental details